Usually no. If you submit a smaller file to a stock agency, you’re simply choosing to license a lower-resolution image. Most agencies expect you to upload the highest resolution you want customers to be able to download, since buyers typically receive the file directly from the agency after purchase.

Resizing the image slightly does not meaningfully protect your copyright or prove ownership. Better evidence of authorship includes your RAW file, related outtakes from the same shoot, capture metadata, and the upload/registration records at the agency.

If your concern is theft, reputable stock agencies already have an incentive to prevent unauthorized use of images on their sites. So the practical question is not “should I shrink it to protect it?” but “what maximum quality do I want to license?” Upload that version, and keep your original RAW and full-resolution files archived for your own records.